1. Field of the Invention
The present invention relates generally to a method for producing a sensing element of an oxygen sensor, and more particularly to a method suitable for fabricating the sensing element so as to improve the operating response of the oxygen sensor, and the adhesion of an undulated layer to a solid electrolyte main body of the sensing element.
2. Discussion of the Prior Art
In the art of controlling an air/fuel (A/F) ratio of an air-fuel mixture for an internal combustion engine for automotive vehicles or other applications, it is known to detect the oxygen concentration of exhaust gases emitted by the engine, by a sensor which uses an oxygen-ion conductive solid electrolyte such as zirconia ceramics. The sensor is operated according to the principle of an oxygen concentration cell.
Such an oxygen sensor for detecting the oxygen concentration employs a sensing element which includes a tubular solid electrolyte body which is closed at one of its opposite ends and open at the other end. The solid electrolyte body has an inner and an outer electrode on its respective inner and outer surfaces. The inner electrode serves as a reference electrode which is exposed to an ambient air as a reference gas having a known oxygen concentration. On the other hand, the outer electrode serves as a measuring electrode which is exposed to a measurement gas in the form of the exhaust gases emitted by an internal combustion engine. According to this oxygen sensor, the concentration of oxygen in the exhaust gases is determined by measuring an electromotive force that is induced between the reference and measuring electrodes, based on a difference in the oxygen concentration between the reference gas and the measurement gas.
In a known oxygen sensor of the type described above, an oxygen-ion conductive solid electrolyte constitutes a suitably shaped main body of the oxygen sensing element, on which the electrodes are formed in contact with the surfaces of the solid electrolyte. In operation, the oxygen sensor is subject to heat of the exhaust gases having a generally high temperature. Consequently the measuring electrode tends to suffer from separation or peel-off from the surface of the solid electrolyte main body, and the sensing capability or measuring accuracy of the oxygen sensor is deteriorated. To solve this problem, U.S. Pat. No. 4,477,487 and German Pat. No. 3118299 propose a technique to improve the adhesive strength between the main body and the measuring electrode, by disposing the measuring electrode on an undulated outer layer of a solid electrolyte formed as an integral outer part of the main body of the sensing element. In order to improve the durability or life expectancy of the measuring electrode, a porous ceramic protective coating is formed on the measuring electrode to protect it from exposure to the exhaust gases.
The undulated outer solid electrolyte layer indicated above is applied to a suitable area on the outer surface of the prepared solid electrolyte main body of the oxygen sensing element, by spraying, dipping or other suitable methods. Subsequently, the formed undulated outer layer is fired into an integral outer covering of the sensing element. Since the density of the unfired solid electrolyte material of the undulated outer layer is lower than that of the solid electrolyte of the pre-formed main body of the sensing element, the fired undulated outer layer tends to have many open pores or voids, in which there exists an atmosphere in an exhaust pipe connected to an engine while the engine is not in operation. When exhaust gases are emitted from the engine and the oxygen sensor is exposed to the exhaust gases, the atmosphere in the open pores is only gradually replaced or interchanged by the exhaust gases in the exhaust pipe. Namely, the replacement or interchange of the atmosphere in the pores takes a relatively long time, and the oxygen sensor suffers from a deteriorated operating response, i.e., increased response time of the sensor output (electromotive force) to a change in the oxygen concentration (oxygen partial pressure) of the exhaust gases as a measurement gas.